Isn't that just a leveler? If yes, you need a compressor to scan the audio material and a gain control which will be triggered by the compressors gain reduction. You can implement a lookahead with a delay unit.

Split your vocal with a spider, one goes into compressor, one into a mixer. Before the mixer you can add a DDL(full wet, no feedback) with 1ms lookahead. Flip the rack, grab the Gain Reduction from the compressor and connect it to Lolth which you connect to the Gain control of the mixer. In Lolth you can now control up or down leveling. The compressor controls the reaction time and ratio.